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Basics of Vacuum Bagging Fins Continued from page 6 Continued on page 8 Join Tripoli.org Mention Apogee Components ApogeeRockets.com/TARC What does peel ply do? Say your surface of the fin is not perfectly smooth, and has some very broad valleys in it. When you lay the fiberglass cloth on the part and add epoxy, and then put it inside the vacuum bag, the upper surface will want to self level out. You’ll end up having a broad puddle of epoxy on the surface that fills the shallow valley. That means you will have excess epoxy on the part, and that adds unnecessary weight. To get rid of this excess epoxy, you can put a slippery cloth on top of the skin. This is the peel ply. This cloth will also follow the curvature of the valley under the skin, and the excess epoxy will pool on top of it. Essentially, nothing changed in this situation, and we still have a pool of epoxy on the surface. But once the epoxy is cured, we can simply pull off the peel ply fabric, and the pooled-up epoxy will come off with it. So we can get rid of a lot of weight by using peel ply. And this is why people think that vacuum bagging will squeeze out excess epoxy. It isn’t that we’re squeezing it out, we’re just removing the large puddles on the surface of the part. It is still totally possible that the cloth is still oversaturated with epoxy that it doesn’t need for strength. A lot of people say that peel ply will leave a smooth surface. The word “smooth” is relative. What they mean is that it will be free of big clumps and hairs of fibers protruding from the surface. Those clumps and hairs are pressed down by the peel ply. But the surface will still have a texture to it. The texture will be the roughness of the cloth used for the peel ply. For example, think of rubbing your fingers across the surface of a cloth umbrella. It is “smooth,” but not “glass-smooth.” The nice thing is that the surface can be lightly sanded and then polished, and it is possible to get it glass-smooth. But it does require extra work. Using peel ply may require only doing one side of the fin at a time. The reason for doing only one side at a time is because the peel ply would be difficult to remove from the edges of the fin if the two pieces of epoxy-filled peel ply touched each other. That’s just my guess though, since I haven’t used peel ply on fins. If you’re using peel ply, you will also need breather/ bleeder cloth over the entire surface. This will both soak up the excess epoxy on the surface of the peel ply, and also allow air bubbles to escape. Both the peel ply and the breather cloth are thrown away after they touch the resin, as they will be clogged up after the epoxy is cured. In my particular application, where I’m trying to get a smooth skin on the surface of the fin, I did not use a peel ply or bleeder cloth directly on the part. I want the fin to touch the side of the bag so the surface is completely smooth once the epoxy is hardened. At this point, the whole thing is inserted into the vacuum bag. Page 7 Issue 569 / Mar. 15th, 2022PDF Image | Basics of Vacuum Bagging Fins
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